Radial keratotomy procedures are currently available for myopic patients having about 2-6 diopters of myopia. Such keratorefractive procedures attempt to flatten the cornea with radial incisions which begin paracentral to the cornea center and leave the central optical zone of about 3-4 mm unaffected. Two widely available techniques currently employed are known as the Russian (up hill) and the American (down hill) methods. In the Russian method, the incision is started in the cornea near the limbus and is directed to the edge of the optical zone. In the American technique, the initial incision is made at the optical zone, and then proceeds radially outward toward the limbus. With both procedures, approximately 4-16 radial incisions are made in the cornea. These incisions are designed to reconfigure the cornea so that the light entering the eye tends to focus more accurately on the retina. However, since the corneal thickness varies from patient to patient, and from one region of the cornea to the next, and since the skill level of surgeons varies from one surgeon to to the next, the predictability of keratorefractive surgery has typically been low. The lack of predictability and efficiency are especially true when employing the American style incisions, as these typically yield incisions of variable depth.
Recently, as disclosed in U.S. Pat. No. 4,691,716, studies have shown that the efficacy of radial keratotomy and the final degree of refractive correction is significantly influenced by the depth of the incision. Procedures which produce a relatively shallow cut are known to produce the least amount of correction with the greatest degree of regression. Accordingly, surgeons must be careful to cut fully through approximately 90% of the thickness of the cornea in order to provide effective results. Since the American style incisions do not provide uniform depth (producing shallower incisions centrally), and since the corneal periphery is thicker (0.580-0.600 mm) than the paracentral and central corneal zones (0.500 mm) in most patients, the surgeon must redeepen the peripheral cut in order to provide an incision at 90% of the corneal thickness over its entire length, following American style incisions.
Redeepening the incision presents the formidable risk of puncturing the cornea and creating an entrance bacteria to enter the anterior chamber of the eye with attendant risks of infection and complications. Redeepening the incision also often requires lifting the blade and retracing or reversing the cutting motion. This is a very difficult procedure and can result in penetration into the optical zone, as well as inaccurate retracing of the bottom of the incision. Incisions invading the full thickness of the optical zone are associated with optical glare, since the resulting surface scarring has a different refractive index than the surrounding corneal tissue. While the Russian (up hill) incisions tend to provide uniform depth with resultant greater predictability, they are still fraught with the danger of invading the central optical zone. This is because slight irregular movements of the patient's eye or the surgeon's hand may result in optical zone incisions as the blade approaches the central zone.
Accordingly, there is a present need for knive blades and keratotomy surgical procedures which permit carefully controlled keratorefractive incisions approaching 90% of the thickness of the cornea without risking puncture into the anterior chamber or scarring at the surface of the optical zone.